(1) Field of the Invention
This present invention relates to an EL (Electro Luminescence) light emitting touch switch having a plurality of touch detection electrodes, and more particularly to an EL (Electro Luminescence) light emitting touch switch suitable for use in a control panel for devices in the interior of an automobile.
(2) Description of the Related Art
So far, there has widely known a touch switch designed to, when a portion of a human body comes into contact with or comes close to a conductive electrode member, detect this state for placing a desired device into operation. Even in a case in which the electrode member functions as an electrostatic capacitance sensor, such a touch switch is made to read a variation in impedance of the electrode when the human body comes into contact with the electrode, thereby detecting the on/off-state of the touch switch.
Meanwhile, as such a touch switch, there has been known a structure in which an electrode designed to detect the contact with a human body is formed on an organic or inorganic EL (Electro Luminescence) element made to emit light from a fluorescent material so as to achieve the switch thickness reduction and the switch size reduction (for example, see Japanese Patent Laid-Open No. HEI 5-135654 (D1)).
FIG. 6 is an illustration of one example of illustrative cross-sectional view showing a structure of such a touch switch, where the reference numeral 100 designates a transparent insulation substrate. Moreover, a transparent electrode 102 is provided on a surface (upper surface) of this transparent insulation substrate 100 and a transparent insulation film (coat) 101 is put thereon, thereby forming a touch surface.
In addition, an EL light emitting layer 103 is formed on a rear surface (lower surface) of the transparent insulation substrate 100. This EL light emitting layer 103 is formed by stacking up a transparent electrode 103a, a fluorescent layer 103b, an insulation layer 103c and a back electrode 103d in a downward direction from the insulation substrate 100 side. The light emitting takes place from the fluorescent layer 103b in a manner such that an alternating-current electrical field is applied between the electrodes 103a and the 103d. 
Since this light reaches the external through the transparent electrode 103a, the transparent insulation substrate 100, a touch detection transparent electrode 102 and the transparent insulation film 101, an operator can touch the insulation film 101 by his/her fingertip by the help of this light emitting. In addition, when the fingertip comes into contact with the insulation film 101, a touch judgment circuit (not shown) connected to the detection transparent electrode 102 senses a variation in impedance or a variation in capacitance so as to judge the occurrence of the touch and further carry out the on-off control on devices (for example, air conditioner and audio device) connected thereto.
However, since such a touch switch composed by the pair of electrodes 103a and 103d, which drives the EL light emitting layer for the light emitting, and the touch detection electrode 102, face difficulty in achieving reduction of the switch thickness and an increase in cost caused by an increase in the number of electrode layers to be stacked up.
Accordingly, for solving these problems, Japanese Patent Publication No. 3284259 (D2) discloses an EL light emitting touch switch capable of achieving the switch thickness reduction by using a touch detection electrode and an EL driving electrode in common.
Concretely, as shown in FIG. 7, a touch switch SW includes an EL light emitting layer 1 configured by piling up a fluorescent layer 1b and an insulation layer 1c between a transparent electrode (which is equally referred to as a “surface electrode”) 1a and a back electrode 1d, and is made by forming the EL light emitting layer 1 on an insulation substrate 3 and further forming a transparent insulating film 2 thereon. In this case, the surface electrode 1 combines a function as a touch detection electrode and a function as an EL driving electrode.
Furthermore, as shown in FIG. 7, a touch judgment circuit 10 designed to detect a variation in the electric potential of the transparent electrode 1a of the EL light emitting layer 1 for detecting the contact of an operator with the transparent insulation film 2 and an EL driving circuit 11 made to output an alternating-current voltage with reference to an electric potential of the transparent electrode 1a for carrying out the light emitting of the EL light emitting layer 1 are placed in parallel on this touch switch SW.
Still furthermore, in addition to the touch judgment circuit 10 and the EL driving circuit 11, this touch switch SW is equipped with a drive control circuit 12, and the touch judgment circuit 10 is operated by a control signal from the drive control circuit 12. The touch judgment circuit 10 is designed to detect the contact of a finger with the transparent insulation film 2 on the basis of a variation in electric potential between an electric potential VA of the transparent electrode 1a and a ground electric potential VG. When the touch judgment circuit 10 has made a touch judgment, it drives an on-off state of the EL driving circuit 11 to apply an alternating-current electrical field between both the electrodes 1a and 1d for controlling the light emitting of the EL light emitting layer 1.
Meanwhile, it is considered to employ such a touch switch in a control panel for equipment (concretely, an audio device or an air conditioner in an automobile) in the interior of an automobile.
However, because the technique disclosed in D1 cannot reduce the thickness, it is not suitable for a control panel in an automobile. Moreover, although the technique disclosed in the Patent Document 2 does not create a problem when the touch detection electrode (transparent electrode 1a) is one in number, in a case in which a plurality of touch electrodes 1a exist on a single panel, for example, a control panel for devices in an automobile, the technique disclosed in the D2 does not function as an effective solving means.
That is, in an EL panel having many independent light emitting portions (in this case, they correspond to the fluorescent layer 1c), a common manner is such that the back electrode 1d is formed to have an area covering all the light emitting portions and is made as a common electrode. However, in the EL panel having this configuration, as disclosed in the D2, in case that an EL driving circuit is connected with reference to the transparent electrode 1a which also serves as a touch detection electrode, that is, the back electrode 1d is configured so as to float with respect to a ground electrode, a large electrostatic capacitance exists at the independent light emitting portions, i.e., between the transparent electrode 1a and the back electrode 1d. When a finger comes into contact with any one of a plurality of touch electrodes, all the switches detect the touch simultaneously (see FIG. 8).